The invention relates generally to towed arrays and more particularly, to vibration isolation modules.
Towing a sensor or an array of sensors through a body of water by a surface or a submerged vessel is a procedure useful for many information gathering operations. However, various environmental and operational factors associated with the towing operation tend to degrade sensor performance by adding to the noise level impacting the sensor. Such factors include noise produced by the motion of the sensor in the water, noise caused by the turbulent flow of water around the towing mechanism, strumming noise along the towing cable, noise from the towing vessel transmitted along the towing cable, and the hydrodynamic whip-like action on the sensor when turning. Some noise takes the form of standing wave energy or vibration energy on the towing cable. A technique used for dissipating this vibration energy involves mounting a vibration isolation module between the array and the tow cable. One type of vibration isolation module has a stretchable cable connected between the sensor and the tow cable to resiliently bear the load. Enclosing the stretchable cable is an outer hose which is also stretchable. Mounted on the stretchable cable and in contact with the outer hose is a spacer formed of a rigid material for transmitting vibration energy. In the case of vibration energy on the outer hose such as that caused by water flow, the spacer will transmit that energy to the stretchable cable for dissipation. In the case of vibration energy on the stretchable cable such as from the towing vessel propagating along the cable the spacer will transmit that energy to the outer hose for dissipation.
A prior technique for mounting the spacer consisted of "hard mounting" it on the stretchable cable. One problem with a vibration isolation module (VIM) constructed in accordance with this technique has been its inability to manage uneven stresses or nonuniform loading imparted to the various components of the VIM during the towing operation. When the load on the VIM is increased such as by increasing the towing speed, certain parts of the VIM may receive different stresses. For example, the outer hose tends to "bag" at the rear due to the drag. Since the parts are in contact with each other, these different stresses can cause failure when the parts are not free to move in relation to each other. For example, where the rigid spacer is hard mounted to the stretchable cable by inserting a pin through both, the spacer is severely restricted in movement in relation to the cable. When the outer hose, which may also contact the spacer, receives a stress different from that on the cable, it will attempt to move the spacer in relation to the hose. This stress is concentrated at the area where the pin is inserted through the cable and has been the cause of failures of the stretchable cable in the past.
Accordingly, it is an object of the invention to provide a vibration isolation module which can better manage the stresses imposed by underwater towing.
Another object of the invention is to provide a vibration isolation module which is constructed so as to allow a certain amount of relative movement between certain of its component parts.